1. Field of the Invention
The invention relates to a correcting device to compensate for perturbations of the polarization distribution over the cross section of a light beam in an optical system, for example a projection objective for a microlithographic projection exposure apparatus.
2. Description of Related Art
A correcting device and a projection objective of this type are known from DE 198 07 120 A1 (the U.S. counterpart of which is patented as U.S. Pat. No. 6,252,712 to Fuerter et al.).
In many optical systems, a prerequisite for a high imaging quality is that the light passing through the optical system should be in a defined polarization state everywhere over the beam cross section. Since this defined polarization state need not be constant over the beam cross section, the term “defined polarization distribution of the light” is also often employed. If deviations from this defined polarization distribution occur, then this can lead to intolerable imaging errors and/or contrast losses in the image plane. Examples of relevant causes of such deviations are the polarization dependency of reflective layers or the birefringence of particular lens materials.
The latter point is of particular importance in connection with microlithographic projection exposure apparatus, such as those used for the production of large-scale integrated electrical circuits. In that case, it is becoming increasingly common to use lenses made of fluorspar (CaF2) since these crystals still have a sufficient optical transparency even at very short projection light wavelengths. At very short wavelengths, however, fluorspar is by its very nature (i.e. intrinsically) birefringent; birefringence due to mechanical stresses may furthermore be added to this. Although various measures for reducing the birefringence of fluorspar lenses are now known, full compensation for perturbations of the polarization distribution over the cross section of the projection light beam is nevertheless generally impossible. If these perturbations remain uncompensated, then the contrast of the projection objectives is reduced, and this has an unfavorable effect on the size of the structures that can then be produced with such apparatus.
The fact that the polarization distribution perturbations considered here have the property of varying over the cross section of a light beam precludes compensation by conventional polarization compensators, for example a Soleil-Babinet compensator, since these compensators act uniformly over the cross section of the light beam.
German Publication DE 198 07 120 A1 discloses a correcting device to compensate for polarization perturbations which vary locally over the cross section of a light beam. The correcting device described therein comprises a plate, which consists of magnesium fluoride (MgF2) and is therefore birefringent, introduced into the beam path of the optical system. The thickness of the plate varies over its cross section, which leads to a position-dependent compensating effect. Since the thickness variations required for the compensation amount only to a few micrometers, the freeform surfaces on the plate cannot be produced by polishing or other conventional methods of material erosion. Production of the freeform surfaces by means of ion beam processing is therefore proposed. Such processing methods are used, for example, in the production of so-called “nano-aspheres” which are used to correct wavefront errors in projection exposure apparatus.
In order to compensate for a class of polarization perturbations which is as general as possible, it is proposed to use two such plates whose principal axes are mutually rotated by 45°. Since the thickness variations affect not only the polarization but also, to an even greater extent, the wavefront profile of light passing through, each of the correcting plates is provided with a quartz plate for wavefront compensation, which also have thickness variations but ones which are complementary with those of the correcting plates. When taken together, a correcting plate and its associated quartz plate, which are assembled together seamlessly by contact bonding or cementing, have scarcely any effect on the wavefront profile of transmitted light because their refractive indices are approximately equal.
A disadvantage with this known correcting device, however, is that the correcting plates do not only influence the polarization where perturbations are to be compensated for, but change the polarization over the entire cross section of the light beam. This is because the correcting plates cannot comprise birefringent material exclusively where compensation for polarization distribution perturbations is required. In that case, the correcting plates would need to be only a few micrometers thick and also have holes at the positions where no perturbations are to be compensated for. Such correcting plates would be neither producible nor manageable. The correcting plates must therefore comprise additional material, which acts as a kind of support but also contributes to the influence on the polarization over its entire cross section.